Zubko Evgenij, Petrov Dmitry, Grynko Yevgen, Shkuratov Yuriy, Okamoto Hajime, Muinonen Karri, Nousiainen Timo, Kimura Hiroshi, Yamamoto Tetsuo, Videen Gorden
Graduate School of Science, Tohoku University, Aoba, Aramakiaza, Aoba-ku, Sendai 980-8578, Japan.
Appl Opt. 2010 Mar 10;49(8):1267-79. doi: 10.1364/AO.49.001267.
There are two widely accepted restrictions on the application of the discrete dipole approximation (DDA) in the study of light scattering by particles comparable to the wavelength: (1) when considering dielectric particles, the size of the cells must satisfy the condition kd|m|<0.5, where k is the wavenumber, d is the size of the cells, and m is the complex refractive index of the constituent material and (2) when considering conductive particles, the size of the cells must be small enough to reproduce sufficiently the evolution of the electromagnetic field in the skin layer. We examine both restrictions when the DDA is applied to irregularly shaped particles and show that its restrictions are not as strong as is widely accepted. For instance, when studying irregularly shaped particles averaged over orientations, even at kd|m|=1, the DDA provides highly accurate numerical results. Moreover, we show that the impact of using large constituent cells is similar to that produced by surface roughness; therefore, the replacement of the target particle by an array of large constituent cells has the same effect, qualitatively, as incorporating additional small-scale surface roughness on the particle. Such a modification of the target particle can be desirable in many practical applications of DDA when irregularly shaped particles are considered. When applying DDA to conductive, nonspherical particles, the insufficient description of the electromagnetic field in the skin layer does not lead to a violation of the Maxwell equations, although it has a visible but nonmajor influence on the light-scattering properties of the target.
在研究与波长相当的粒子的光散射时，离散偶极子近似（DDA）的应用存在两个被广泛接受的限制条件：（1）当考虑电介质粒子时，单元尺寸必须满足kd|m|<0.5的条件，其中k是波数，d是单元尺寸，m是组成材料的复折射率；（2）当考虑导电粒子时，单元尺寸必须足够小，以便充分再现趋肤层中电磁场的演变。我们研究了将DDA应用于不规则形状粒子时的这两个限制条件，并表明其限制条件并不像广泛认为的那样严格。例如，在研究取向平均的不规则形状粒子时，即使在kd|m| = 1的情况下，DDA也能提供高度准确的数值结果。此外，我们表明使用大的组成单元的影响与表面粗糙度产生的影响相似；因此，用大的组成单元阵列替代目标粒子在定性上与在粒子上加入额外的小尺度表面粗糙度具有相同的效果。在考虑不规则形状粒子的DDA的许多实际应用中，对目标粒子进行这样的修改可能是可取的。当将DDA应用于导电的非球形粒子时，趋肤层中电磁场描述不足不会导致违反麦克斯韦方程组，尽管它对目标的光散射特性有明显但非主要的影响。
